Chronic hepatitis, which can be of alcoholic, viral or other origin, has a fibrotic effect that is important to evaluate to determine the best time to treat the hepatitis.
There do not exist on the market at present devices for measuring the elasticity that can be performed in a noninvasive manner, i.e., without collecting a portion of the organ or medium.
U.S. Pat. No. 5,882,302 is known in the prior art. It describes a transducer attached to a motor. The motor enables displacement of the transducer in a manner to obtain images of the different zones of the medium. The motor is thus used to modify the imaged zone and not for generating a low-frequency impulse. Moreover, the displacement in this context is not parallel to the axis of the ultrasonic beam.
Also known is U.S. Pat. No. 6,277,074 which describes a device in which displacement of the motor is also parallel to the ultrasonic axis. It does not disclose acquisition of the signals during compression. In fact, as in the case of U.S. Pat. No. 5,882,302, the motor is used to displace the transducer and not for generating a low-frequency impulse.
U.S. Pat. No. 5,099,848 discloses an ultrasonic device associated with a vibrator used in monochromatic mode of frequency fixed at 50 Hz. Moreover, in that device the transducer is not carried by the actuator and thus cannot be used for generating a low-frequency impulse.
With regard to the most recent devices for the study and analysis of the elasticity of a medium. WO 00/55616 describes an imaging method for observing the propagation of a low-frequency shear impulse wave simultaneously at a multitude of points of a diffusing viscoelastic medium. For this purpose there is emitted an ultrarapid cadence of ultrasonic compression waves which enable production of a succession of images of the medium. The images obtained in this manner are then processed by intercorrelation to determine at each point of each image the movements of the medium upon propagation of the shear wave. This device does not enable localization of the zone in which the elasticity is measured.
In available devices, when the ultrasonic transducer is used to generate a low frequency impulse by vibrating mechanically, the transducer is mobile and the reference frame is not fixed. One uses a technique that is well known by the expert in the field to compensate for this displacement. That solution has multiple drawbacks:                it uses an ultrasonic echo originating from a deep and immobile zone of the medium,        it has low precision because the medium is not immobile, the form of the low-frequency impulse is poorly determined,        it represents a supplementary algorithm that increases the calculation time,        the surface of the medium presents a resistance to the applied shock, the real form of the low-frequency impulse, depends on the pressure applied by the operator.        
In addition to the problems linked to the compensation of the displacement of the vibrator, the pressure exerted by the operator is a parameter that is not taken into account when it disturbs the measurement of elasticity.
Moreover, the study of shallow media with a system of the conventional type in direct contact can be difficult because the focal zone of certain transducers does not enable production of a clean ultrasonic signal at a short distance from the transducer.
In the measurement of conventional displacements implemented, e.g., by blood flows, the amplitude of the displacements is not linked to the depth in the medium, but to the phenomena observed, e.g., the displacements linked to the flow of blood are greater in the center of the artery than at its sides. The algorithm used to measure the displacements is thus the same irrespective of the depth. In contrast to elastography, the amplitude of the displacements depends on the distance which was given to the low-frequency vibration. When the vibration was given from the surface, the amplitude of the displacements generated by the low frequency impulse decreases as the wave penetrates deeply into the tissues. The use of a classic algorithm is not favorable for the measurements of the displacements over the entire range of depths.